Magnetic recording heads typically comprise a transducer and a slider. These are supported in proximity to a magnetic recording medium, usually a spinning disk with a magnetic coating, by a suspension assembly. The suspension comprises a load beam that attaches to the slider and read/write transducer assembly via a flexible gimbal device at one end and a flexible section of the suspension, namely the formed area, at the other end. The formed area in turn is connected to a suspension support arm. The suspension support arm connects to an actuator. It is desirable for the transducer suspension to be flexible in a direction perpendicular to the plane of the disk so that the suspension is able to follow any movement of the head due to disk run out, or wobbling of the disk normal to its plane. If the transducer does not follow the out of plane motion of the disk, head to disk spacing variations will result. Additionally, the suspension should be extremely rigid in a plane parallel to that of the disk so the transducer can be accurately placed over a data track. The slider and the magnetic element are positioned over the proper track of data by a voice-coil powered actuator in order to read, write, and erase data.
Data is transferred to a magnetic recording medium in the form of magnetic flux reversals from a gap in the transducer. Recently, advances have been made that allow very high densities of data to be stored on a single disk. For accurate and dense data encoding, the read/write gap in the transducer must be maintained as close to the disk as possible at a constant height; typically about 6 micro-inches above the disk. The storage capacity of the disk is a strong function of the height of the read/write gap above the disk, i.e., the flying height. Storage capacity is also a function of the track density, i.e., the number of recording tracks available radially. This is determined by the accuracy with which the actuator motor can locate the transducer over a previously written track of data and follow that track as the disk spins. Therefore, the storage capacity of the disk is measured by the number of flux reversals per square inch. More precisely, the area density is calculated by the linear bit density, or the number of flux reversals per inch along a track, times the radial track density, or the number of tracks available radially. Typically the linear bit density is an order of magnitude greater than the radial track density. The number of flux reversals per inch is extremely sensitive to the head to disk interface spacing; one micro-inch of flying height variation significantly reduces the number of flux reversals per inch. Hence, it is very important to keep the head to disk spacing as constant as possible.
Fluctuations in the flying height, most commonly caused by head oscillations due to suspension resonances, degrade the performance of the disk drive by increasing the length of flux reversals. This decreases the voltage, or amplitude, of the data pulse that is read by the transducer, thus decreasing the signal to noise ratio. Suspension resonances are generally excited in the suspension during data seek and track following operations. However, there are also other sources that can cause the suspension to resonate, including external disturbances. In order to maximize the capacity of disk drives, it is necessary to control the resonant behavior of the suspension such that the suspension does not cause the head to disk spacing to vary significantly when the actuator is active.
In the powered down state, the read/write transducer is held against the disk by a preload force provided by the formed area of the suspension. The formed area is a flat spring which has a zero load position in a plane below the disk, therefore, when the transducer is in the plane of the disk, the formed area forces the transducer to remain in contact with the disk. During operation, a boundary layer of air is carried along the surface of the rotating disk. This boundary layer is generated by viscous forces in the operating fluid (air). Since the transducer is a hydrodynamic air bearing, i.e., it has very flat, highly polished surfaces in contact with the disk, the viscous operating fluid gets trapped between the transducer and the disk and forces the head to fly above the disk when the disk is spinning. Ideally, the formed area of the suspension should provide a preload force that is equal, but opposite in direction, to the hydrodynamic forces generated by the transducer when the read/write element is flying at the design flying height. At the same time, the formed area must be compliant enough to allow for minor variations in the displacement of the read/write element while not changing the preload force on the transducer. That is, the formed area's spring constant, or force per displacement, should be very small such that the change in preload force varies only slightly as the head follows the run out in the disk and also during the drive assembly process.
One very significant factor which influences dynamic transducer flying height is magnetic recording head suspension resonance. There are other factors which affect the flying height of the transducer, however, these are quasi-static variations, i.e., variations which occur at one third the frequency of the first resonant mode of vibration of the system, and are not caused by the dynamic behavior of the suspension. During suspension resonance, the transducer element is forced to modulate, causing a significant decrease in the signal to noise ratio of the system, as well as other detrimental phenomena previously mentioned. Another undesirable effect of suspension resonance is transducer off track error. That is, due to the geometry of the gimbal spring, which attaches the read/write element to the suspension, the transducer moves across tracks, leading to error in positioning the magnetic recording head. This behavior not only reduces the track density, because wider tracks must be designed for in the event of cross-track error, but also interferes with the electronic data-track following capability of the feedback system. During suspension resonance, cross-track modulations of the transducer can be so extreme that the track following electronics, which control the actuator, can become confused causing the mechanical system to become unstable. Ultimately, actuator instability leads to violent oscillations of the transducer suspension, causing the head to contact the disk and possibly destroy the data.
Three types of vibrational modes that commonly appear in magnetic head suspension assemblies are bending modes, torsional modes, and a lumped-parameter mode. The bending and torsional modes are continuous system modes, i.e., modes which can best be modeled by an oscillating string. For example: The first bending mode, associated with a particular frequency, takes on a half-sine wave shape along the longitudinal axis of the suspension, while the second bending mode, associated with another frequency, takes on a full-sine wave shape, and so on. Similarly, the first torsional mode takes on one full twist along the longitudinal center line of the suspension at one frequency, while the second torsional mode takes on two full twists down the center line of the suspension at a different frequency and so on. While the bending and torsional modes have many harmonics, the lumped parameter mode only exists at one frequency in the system. This is due to the discrete nature of the suspension head assembly during this mode of vibration; the entire suspension appears to be a single oscillatory spring while the transducer acts as a single lumped mass. Therefore, this mode behaves as a simple discrete spring/mass system, having only one mode of resonance, occurring at one frequency.
The bending modes of vibration cause the transducer to be displaced in the data track direction, i.e. along the track. Also, the bending modes cause the head to disk spacing, or flying height, to modulate at the resonant frequency. As previously discussed, flying height modulation causes many undesirable phenomena to occur. The torsional modes of vibration cause the transducer to modulate in the cross-track direction, causing data tracking errors. Such errors inhibit the control system from accurately placing the head over a data track, thereby interfering with the operation of the device. The lumped parameter mode also causes the head to modulate in the cross-track direction, but with much greater amplitude than that of the torsional modes. None of the detrimental effects caused by these modes is tolerable in disk drives designed for high density recording.
Often, the vibrational modes are initiated by activating the actuator motor. The actuator uses a closed loop feedback system to locate the suspension, and hence the slider, over the proper track of data. The closed loop feedback system can utilize either a dedicated disk containing only position information to control the location of the actuator, or feedback information can be embedded in each disk such that the transducer in use feeds back position information to the actuator to control an individual head on a particular disk on demand. Position information, either from a dedicated or embedded source, is fed back to the actuator control system. This information forces the data head(s) to remain on track while reading and writing information. In many systems, the actuator control system uses a voice-coil motor to drive the suspension and slider over the disk. When it is desired to position the head at a specific location, the motor is driven by a voltage that has a very short rise time, such that the actuator is accelerated very quickly. Once the actuator is in motion, the voltage levels off and the actuator approaches a constant velocity. As the actuator approaches the proper location on the disk, a similar, but inverse voltage pattern is applied to the voice-coil motor to stop the suspension actuator. Both the voltage rise, to start the actuator, and the voltage drop, to stop the actuator, are best represented by a square wave. Fourier analysis, or frequency domain analysis, of this full square wave reveals that it is made up of a multitude of sinusoids at different frequencies. Thus, movement of the actuator can excite all vibrational modes of the suspension and as a result, cause a combination of modes to be excited at one time. Hence, the suspension must be very well behaved from a dynamic point of view, otherwise, the storage capacity and performance of the disk drive will be seriously reduced.
In summary, the dominant resonant oscillation modes which can adversely affect the suspension are: bending modes, torsional modes, and the lumped parameter mode. The bending modes cause the slider to move normal to the plane of the disk, resulting in variable size data flux reversals and the increased likelihood that the head will contact the disk. The torsional modes and the lumped parameter mode can cause the slider to modulate across the data tracks, i.e., in the cross-track direction of the disk, causing positioning error. As discussed above, any suspension motion which causes the head to modulate in or out of the plane of the disk reduces the information storage and retrieval capacity of the disk drive.
A traditional approach used to overcome problems associated with resonant vibration in the suspension system is to add a constraint layer damper. The constraint layer damper is a thin visco-elastic polymer that is affixed to the load beam to absorb oscillatory mechanical energy in the suspension. As the name suggests, visco-elastic materials possess both viscous and elastic properties. Visco-elastic material is best modeled by a coil spring with a viscous fluid damper in its center. When the spring traverses a displacement, energy is absorbed by the working fluid in the damper passing from one chamber to another through an orifice. When the force which caused the displacement is removed, the spring returns the system to its original state. Although the constraint layer damper functions on the same principle, energy is not lost through compression or expansion of the damper, but rather shear, or parallel relative motion of planes of the material.
Visco-elastic dampers are manufactured by sandwiching a sheet of visco-elastic material between one steel constraining layer which is cut to the proper shape and the object to be damped. Visco-elastic material is inherently sticky, giving an intrinsic adhesive backing. Use of an adhesive backed visco-elastic material in a production environment requires additional handling of the suspension and therefore, additional expense. Other difficulties exist as well, including contamination, damper placement variability, damping temperature dependance, and the necessity for additional thermal cycling. Contamination is due to the fact that visco-elastic materials attract dust and other microscopic particles, which can be released into the disk drive. Damper placement on the suspension must be carefully controlled to insure the visco-elastic material is located in the correct area of the suspension. The visco-elastic properties of the damper also vary with temperature. At high temperature, the constraint layer damper looses its damping qualities, allowing too much vibration in the suspension, while at low temperature the damper becomes extremely elastic and hence is unable to damp oscillation. Additionally, the damper requires further handling in that it necessitates thermal cycling to relax the visco-elastic material such that it conforms to the shape of the preloaded suspension. This is the only way to insure that the damper does not cause an off tracking error due to the damper relaxing when the drive operates for the first time. Finally, the change in the properties of the damper as it ages is not well understood.